Chromium sites in zeolite framework: Chromyl or chromium hydroxyl groups?

A combined experimental and ab initio periodic DFT calculation on the structure and stability of different chromium-substituted zeolite models are presented. Experimental investigation by XRD, FT-IR and UV–vis show the presence of chromium in SiBEA zeolite. The favorable chromium sites are characterized by their calculated geometrical parameters, energetics, vibrational frequencies and protonation/deprotonation energies. The theoretical vibrational frequencies are compared with experimental FT-IR measurements. A phase diagram was constructed on the basis of calculated total energies of the optimized structures. Cr(III) is stabilized in the zeolite framework as a tri-coordinated site. Cr sites are predicted to be more acidic than group V containing zeolites. Dehydration of the site leads to the formation of a Cr(VI) containing a CrO group. These results are fully consistent with experimental data and allow the identification of the molecular structure of chromium sites in a zeolite framework.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Isomorphous substitution of Cr into the zeolite structure is endothermic. ► In hydrated conditions tetrahedral Cr(III) and a bridging hydroxyl are stabilized. ► In dehydrated and oxidative conditions penta coordinated Cr(VI)O site is stabilized. ► Only two different types of Cr(III) and Cr(VI) model sites dominate.